1. Field of the Invention
This invention relates to the field of control and actuation of automatic transmissions for motor vehicles. More particularly, the invention pertains control of pressure in an hydraulic system supplied from multiple hydraulic pumps.
2. Description of the Prior Art
Conventionally, hydraulic systems of automatic transmissions have conventionally been supplied with hydraulic fluid drawn from a sump by a fixed displacement pump. Pumps of this kind are selected so that they produce sufficient flow to match the requirements of the hydraulic system at low speed, generally at the idle speed of the engine. At higher speed, the pump produces flow greatly in excess of the requirements of the hydraulic system. Excess flow is diverted directly to the sump causing low operating efficiency of the system. To avoid this difficulty, variable displacement pumps have been substituted for the fixed displacement pumps so that the flow capacity of the pump matches the pump speed. Difficulty has been experienced in controlling instability of variable displacement pumps in this operating environment.
The flow rate of hydraulic fluid supplied to the hydraulic circuit of an automatic transmission depends on many factors and varies in magnitude over a wide range. Fluid is used throughout the transmission to fill and engage friction clutches and brakes, and for lubrication. Pressure for this fluid is maintained at line pressure by a main regulator valve that responds to the magnitude of a control pressure. Except at low speed and under transient conditions, the required volume of fluid at line pressure is typically less than 40% of the pump capacity; therefore, 60% of the work done by the pump can be wasted. Multiple pumps provide a way to decrease these losses by shutting off one pump when flow requirements permit.
Multiple fixed displacement pumps have been combined to supply hydraulic fluid to the hydraulic system of an automatic transmission, one of the pumps having a high displacement and the other a lower displacement. It is necessary that line pressure be regulated uniformly, regardless of whether the circuit is supplied from one or both pumps concurrently.
Cavitation is a condition in which the available fluid at a pump inlet is insufficient to fill the inlet or pumping chambers. Bubbles produced by cavitation implode as they are exposed to system pressure at the outlet of the pump. The condition is the result of excess pump speed, restriction in the inlet line, low level of fluid in the sump or reservoir, or high oil viscosity.
The maximum vacuum at the inlet of a pump should be no more than five inches Hg. Ideally, there should be no vacuum, or even a slight positive pressure, at the inlet; otherwise, cavitation can occur. Cavitation causes erosion of the metal within the pump and speeds deterioration of the hydraulic fluid. A cavitated pump makes a very distinctive noise as the bubbles implode under pressure.
One way to avoid cavitation is to pressurize the pump inlet such as by locating the reservoir sump above the pump. However, in an automatic transmission, this location of these components is difficult to accomplish. An auxiliary pump can be used to maintain the supply of oil to the inlet at low pressure.